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Oriental Bittersweet: A Patient Invader

June 24, 2004

Asheville, NC — USDA Forest Service research on oriental bittersweet confirms
suspicions about the plant's destructive invasion of the forests of
Southern Appalachia. Studies by Southern Research Station (SRS)
scientists at the Bent Creek Experimental Forest near Asheville, North Carolina provide an understanding of the unique Âsit and waitÂ strategy adopted by the destructive vine.

Oriental bittersweet (Celastrus orbiculatus), a woody vine
with rounded leaves and small yellowish flowers, was introduced to the
United States from Southeast Asia around 1860. The bright orange
berries produced in the fall have made oriental bittersweet popular for
wreaths and winter flower arrangements, but the pretty vine wreaks
havoc on the trees and native plants of the Southern Appalachian
forest. The vine can spread by root suckering, but is primarily
dispersed by the birds and mammals that eat the berries - and sometimes
by people using the vines to decorate. Oriental bittersweet easily
proliferates in forest openings created by disturbance.

Oriental bittersweet on mature white pine. Photo: Z. Hoyle

Asheville, North Carolina, is a hub for oriental bittersweet
invasion. The vine is literally moving out along roads and rivers into
the public lands that surround the city, and poses a real threat to
forest trees and plants. Oriental bittersweet grows fast: the plant can
cover tall trees in a season, causing them to collapse from the weight
of the vines. Understory plants are smothered by the vines themselves
or by lack of light.

The native version of the vine, American bittersweet (Celastrus scandens),
looks very similar to oriental bittersweet, except that it flowers and
produces berries at the end of stems, while oriental bittersweet
produces berries where leaf and stem intersect. Not aggressive or
particularly invasive, American bittersweet itself is under threat.
Because it hybridizes so easily with oriental bittersweet, the genetic
integrity of the native plant may be lost.

Cathryn Greenberg,
research ecologist with the Bent Creek unit, has studied the unique
strategy that allows oriental bittersweet to spread so rapidly. In
2001, research by Greenberg, Lindsay Smith (University of Tennessee),
Douglas Levey (University of Florida), and Evelyn Konopik, a German
graduate student working with the National Forests of North Carolina,
confirmed the steady spread of oriental bittersweet out of Asheville
along the Blue Ridge Parkway .

Using greenhouse experiments, Greenberg, Smith and Levey looked at
different factors that might affect how well bittersweet seeds
germinate. They found that bare seeds - those with the flesh and pulp
removed - had the highest germination rate. The researchers also looked
at scarification of the seed covering. The seeds of many plants have a
hard seed cover that must bebroken or scratched before germination can
begin. Some seeds must literally be etched by the gastrointestinal
acids in the stomachs of birds and mammals. ÂAlthough birds are thought
to be the primary dispersers of oriental bittersweet, no one had looked
at whether scarification contributed to germination,Â says Greenberg.
ÂWe fed seeds to captive birds, and somewhat surprisingly, found no
difference in germination rates from seeds that were just defleshed.
But the intact fruits with flesh did take longer, suggesting that
getting eaten by birds does help the seeds to germinate.Â

Experiments manipulating available light showed that light intensity
did not affect the proportion of seeds germinating, the time until
germination, or seed survival. ÂWe found a high level of germination
over a wide range of conditions,Â says Greenberg. ÂOur results confirm
that bittersweet seeds are dispersed in large numbers, and that the
plant can readily establish and persist in low light under the forest
canopy. When a hole in the forest canopy allows light to reach the
ground, the plants begin growing rapidly.Â Most invasive plants move
into disturbed sites with high light and reduced competition from other
plants. The unusual Âsit and waitÂ strategy of oriental bittersweet
allows the plant to slowly invade an intact forest and wait for a
canopy disturbance to spread rapidly.

To learn more about the ecology of oriental bittersweet in forested settings, Henry McNab, researcher forester with the Bent Creek unit and project leader David Loftis
analyzed different sites in relation to occurrences of the plant. They
found that the presence of bittersweet was associated with moist areas
with mature trees and few shrubs. Bittersweet generally did not grow
where the forest canopy was dominated by oaks or where there was no
bare soil exposed. Oriental bittersweet was also absent from sites with
mountain laurel, which tend to be dry.

Unfortunately, very little is known about how to get rid of oriental
bittersweet. In the article published about the site research, McNab
recommends that managers of lands invaded by oriental bittersweet start
by aggressively controlling isolated patches of the vine. Greenberg
recommends removing the vines before the fruit forms to minimize the
spread of seeds by birds, animals, and people. Vines can be removed
fairly easily by hand pulling and clipping, but removing them from
highly infested areas is very labor intensive.

During July and August, the Bent Creek unit will start a measured
attack on oriental bittersweet. The Forest Service will work with
community volunteers trained by the North Carolina Exotic Pest Plant
Council and the Asheville Weed Team to clear the invasive vine from
sections of the Bent Creek Experimental Forest. Click here for more details.

Thoroughly wet all leaves with one of the following
herbicides in water with a surfactant (July to October): Garlon 4,
Garlon 3A, or a glyphosate herbicide as a 2 percent solution (8 ounces
per 3 gallon mix).

For stems or vines too tall
for foliar sprays, apply Garlon 4 as a 20 percent solution in
commercially available basal oil, diesel fuel, or kerosene (2.5 quarts
per 3 gallon mix) with a penetrant (check with herbicide distributor)
to the lower 16 inches of stems. Or, cut large stems or vines and
immediately treat the cut surfaces with one of the following herbicides
in water with a surfactant added: Garlon 4 or glyphosate herbicide was
a 25 percent solution (32 ounces per 1 gallon mix.).

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